Dioscorea, also known as “wild yam,” is a member of the monocotyledonous family Dioscoreaceae, which is distributed in the tropical and subtropical regions. There are about 650 species in the world, of which 93 species and 9 varieties are found in China, and 14 species and 5 varieties are found in Taiwan. Dioscorea is one of the very important pharmaceutical plants used in the traditional Chinese medicine. In the Chinese pharmacopoeia, the medicinal uses of Dioscorea rhizome are prescribed for indigestion, anorexia, diarrhea and diabetes. Dioscorea sp. has many superior characteristics in the rhizome such as high viscosity and high contents of viscous polysaccharide comprising carbohydrate, mannose, arabinose, glucose, galactose, xylose, and rhamnose, starch, protein, vitamins and minerals. It also contains dioscin, diosgenin, phytic acid, allantoin, dopamine, batatasin, dioscorea-mucilage B, and sterols. Certain studies indicate that the unit structure of polysaccharide prepared from Dioscorea batatas is β-(1→4)-linked D-mannose residues containing C-3 branch chain, and the average molecular weight of polysaccharide obtained from Dioscorea japonica is about 81,000.
Dioscorea has been studied for years for their medicinal effects. For example, in Biochemical & Biophysical Research Communications 207(1): 398–404, 1995 February, J. L. Beneytout et. al. reported that, the steroid of Dioscorea, diosgenin, induces morphological and biochemical changes characteristic of megakaryocyte cells when diosgenin is added to human erythroleukemia (HEL) cell cultures, and thus, diosgenin can be used as a megakaryocytic differentiation inducer of HEL cells. In Life Sciences, 59(11):147–57, 1996, a steroid extract of Dioscorea was indicated to possess significant activities as an antioxidant to modify serum lipid levels. In Planta Medica. (3):168–71, 1986, the effect of glycans of Dioscorea on hypoglycemic activity in normal and alloxan-induced hypoglycemic mice have been discussed.
In recent years, people make many efforts to study the medicinal effect of Dioscorea polysaccharide. In Journal of China Pharmaceutical University, 25(6):369–72, 1994, it was reported that Dioscorea polysaccharide decreased, in vitro, the NADPH-Vc induced and cysteine-Fe2+ induced Malondialdehyde formation of brain, liver and kidney microsome in rats, and scavenged superoxide radicals generated by hypoxanthine/xanthine oxidase reaction system and Fenton reaction system. Therefore, Dioscorea polysaccharide seems to be used as an antioxdant and superoxide radical scavenger.
In Journal of Plant Resources and Environment, 5(2):5–8, 1996, it illustrated that the content of polysaccharide and allantoin of Dioscorea tuber have remarkable effect on lowering the sugar and lipid levels in blood.
In HENAN TCM, 16(6):349–350, 1996, the effect of polysaccharide extract of Dioscorea sp. on immunological function was studied, wherein the polysaccharide extract was stated to be prepared by following steps: alcohol reflux, heat extraction with water, concentration, deproteinization by trichloroacetic acid, precipitation by adding 8×, 95% alcohol, dissolution in water, dialysis and dryness. From the given Examples and data, it is showed that the obtained polysaccharide can improve the phagocytosis of macrophage, hemolysin formation, lymphocyte conversion and enhance T cell percent in peripheral blood.
Other studies concerning water extract of Dioscorea, prepared by water extraction and alcohol precipitation, can be found in HENAN TCM, 12(1):23–24, 1992. In that article, the water extract was indicated to be able to improve the hemolysin formation, lymphocyte conversion and to enhance T cell percent in peripheral blood.
In accordance with this invention, it provides an oral composition comprising a Dioscorea polysaccharide extract. The polysaccharide extract is capable of improving the immunological activity by regulating the cytokine gene expression.
Further extensive study showed that Dioscorea polysaccharide prepared by the method of this invention remarkably enhances the effect of oral vaccine on systemic and mucosal responses of a subject upon vaccination with an oral vaccine. More specifically, Dioscorea polysaccharide has an effect on induction of the systemic IgG and mucosal IgA antibodies specific to the oral vaccine. This synergistic effect of Dioscorea polysaccharide on immunization with an oral vaccine is neither disclosed nor suggested in the prior art.